Friends,
I have spent a fair amount of time trying to find a solution to one
of our research problems. There are metamorphosed
Paleoproterozoic sills (basaltic) associated with turbidites, along
an assumed rifted margin in what is now the Black Hills region of
South Dakota. Many of the sills contain the assemblage:
actinolite+quartz+/-chlorite+/-epidote+/-clinozoisite+variable
amounts of calcite. Note that plagioclase is quite rare and that the
amount of calcite is greater near contacts with the
metasedimentary country rocks. Quartzites and phyllites
constitute the vast majority of metasedimentary material.
Carbonate rocks are almost nonexistent. The phyllites contain
white micas, chlorite and only rarely, biotite porphyroblasts.
Our problem is two fold. A) What reaction removed calcic
plagioclase (it is quite common in some other sills in the region)?
and B) Where did all the calcite come from? We have done a
moderately good job of searching literature on lower-grade
metamorphism of metabasites. I would like to account for the two
issues with one reaction or set of reactions, but nothing
satisfactory has yet been found. Unfortunately, we do not have
mineral chemistry on the assembled phases. Any insights on the
situation will be greatly appreciated. Thanks for your help.
Jeff Greenberg
Jeffrey K. Greenberg, Professor and Chair
Dept. of Geology and Environmental Science
Wheaton College, Wheaton, IL 60187
(630)752-5866
Quote 1, "I never would have seen
it, if I hadn't believed
it."
Quote 2, "Nothing is impossible for
the man who doesn't have
to do it himself."
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